1. Field of the Invention
The present invention relates to a brake system which is used in an automobile or the like, and particularly to an electrically powered brake system electrically generating a braking force and a control unit performing a failure diagnosis of the electrically powered brake system.
2. Background Art
As a fail-safe for a failed wheel which suffered a failure of a control system including software and hardware in an electrically powered brake using a motor as a braking force source, fail-open functions of removing a braking force of the failed wheel to prevent a vehicle from being braked have been proposed. The mechanical specifics of these functions are disclosed in Patent Document 1 (JP Patent Publication (Kokai) No. 2003-14014) and others. They are based on a method in which an electrically powered brake actuator is made to completely stop control and input of electrical energy is blocked to remove a braking force.
[Patent Document 1] JP Patent Publication (Kokai) No. 2003-14014
However, the operation of the fail safe described in Patent Document 1 is actuated only in a state in which a failure is detected and controllability becomes worse when brake control is performed during activation of a brake system (including a time during which a vehicle is driven). In an automobile, generally, each of four wheels is equipped with a brake. In case where a braking force is generated on only a specific wheel while a vehicle is driven, a yaw moment is generated in the vehicle with an axis being the wheel on which the braking force is generated. This yaw moment may bend a traveling direction of a vehicle depending on the speed of the vehicle and the situation of a road surface.
In an electrically powered brake system, how a brake can be released without generating a false thrust force due to a failure or a malfunction is an important point for whether or not the traveling direction of a vehicle can be controlled according to a handle operation by a driver.